This invention relates to a method of providing improved prints (images on media) by a printer from images captured by a capture device over a wide variety of exposure conditions.
It is important in producing displays or prints from digital images to fit the dynamic range of the originally captured scene to the dynamic range of the materials available for displaying or printing using the best possible transform. A limiting factor is the dynamic range of the materials available for displaying or printing. By selectively choosing the proper transform for displaying or printing, it is possible to display or print aesthetically pleasing images on materials that have less dynamic range than the original scene.
As new materials such as thermal print media and ink jet technologies have become available for printing, they have introduced a larger dynamic range than available with previous technologies. It is well known how to print digital images on these types of media. However, it is difficult to design different transforms that best take advantage of these new media. A key element in any new transform is to be able to utilize the full dynamic range of the media for displaying or printing the digital images.
There have been a number of techniques for improving the tone scale of digital images, see for example, U.S. Pat. No. 4,792,518 and U.S. Pat. No. 5,300,381. For a discussion of tone scale, see xe2x80x9cThe Tone Reproduction of Colour Photographic Materials,xe2x80x9d R. W. G. Hunt, I. T. Pitt, and P. C. Ward, J. Photog. Sci., 17:198(1969).
As set forth in the above disclosures, the techniques for making the images are very complex and require that the media be photographic media. The publication by Hunt et al describes the xe2x80x9cideal systemxe2x80x9d for printing photographic images to correct for camera flare, printer flare, and viewing flare but offers no practical way to implement this theoretical tone reproduction curve because of the lack of digital imaging tools and the limitations of the materials available in 1969.
U.S. Pat. No. 5,528,339 discloses techniques for improving the tone reproduction of digital images on other media such as thermal, ink jet and electrophotographic. However, the media available for the printing of the digital images far exceeds the dynamic range previously described by having lower minimum densities and considerably higher maximum densities.
Heretofore, in digital image processing, each image is processed separately. By that is meant the scene is calibrated and individually tone scaled for that one image. This, of course, is a time consuming process and is a major drawback in the use of digital images captured by a typical consumer. This is especially true when it is realized that a typical amateur photographer (using film or digital cameras) captures images over a wide photographic space and under different exposure conditions which result in images which may be either under or over exposed compared to a normal exposure.
Digital images are often stored in a xe2x80x9crenderedxe2x80x9d image space, such as sRGB a default RGB color space, where the relationship between the image code values and the scene luminance values is very non-linear. These images may contain exposure and tone scale errors, where the key objects are lighter or darker than desired by the user, due to imperfect exposure determination algorithms in the digital camera or film scanner which created the digital image. Many imaging applications, such as Adobe Photoshop or Kodak Picture Easy, allow the user to adjust the xe2x80x9cbrightnessxe2x80x9d, xe2x80x9ccontrastxe2x80x9d, and/or xe2x80x9cgammaxe2x80x9d of the image by sliding using one or more xe2x80x9cslidersxe2x80x9d controlled by a mouse. The controls adjust the slope, x-intercept, or exponential function of a look-up table that is applied to the image, in order to modify the tone scale of the image when it is displayed or printed. However, because they do not directly adjust the scene exposure, they do not properly compensate for camera exposure or tone scale errors. While it is possible to somewhat improve the image using such controls, this is extremely difficult for unskilled users to determine how to best set these multiple controls.
What is needed is a method that provides proper compensation for camera exposure and tone scale errors which is extremely simple for a non-expert user to understand and utilize.
It is an object of the present invention to facilitate the printing of over and under exposed digital images captured by a capture device over a wide exposure range and which are to be printed by a printer to provide images with improved exposure and tone scale to provide prints with improved exposure correction and improved tone scale.
It is another object of the present invention to facilitate the use of digital image processing by users, reducing their time to produce high quality images.
These objects are achieved by a method for correcting for exposure in a digital image which was captured by an image capture device and which is to be printed on a printer which forms monochrome or color images, on a medium, comprising the steps of:
a) providing a plurality of exposure and tone scale correcting transforms, each such transform being unique to an exposure condition and which corrects exposure and tone scale for a digital image captured by the capture device for such unique exposure conditions and to be printed by the printer; and
b) selecting the appropriate transform from the plurality of transforms and using such selected transform to transform the digital image and produce a new digital image which is corrected for exposure and tone scale when printed from the printer.
It is an advantage of the present invention to provide an improved print that takes into consideration the operating characteristics of the capture device and the printer and corrects for exposure and tone scale by using a plurality of predetermined transforms which a user can readily choose to transform a digital image from a predetermined capture device and apply such transformed image to a printer to provide an aesthetically pleasing print.
It is also a feature of the invention that color correction can readily be accomplished by using color management techniques on a transformed digital image prior to printing. This technique can be readily implemented in a computing system. Such a computing system can include a microprocessor which can apply tone scale and exposure correction transforms which allows a printer to produce continuous tone prints which are aesthetically pleasing.